Multi-channel optical signals typically comprise a plurality of spectral channels, each having a distinct center wavelength and an associated bandwidth. The center wavelengths of adjacent channels are spaced at a predetermined wavelength or frequency interval, and the plurality of spectral channels may be wavelength division multiplexed to form a composite multi-channel signal of the optical network. Each spectral channel is capable of carrying separate and independent information. At various locations, or nodes, in the optical network, one or more spectral channels may be dropped from or added to the composite multi-channel optical signal, as by using, for example, a reconfigurable optical add-drop multiplexer (ROADM). Reconfigurable optical add-drop architectures are disclosed in commonly assigned U.S. Pat. Nos. 6,549,699, 6,625,346, 6,661,948, 6,687,431, and 6,760,511, the disclosures of which are incorporated by reference herein.
An optical switching node may comprise one or more wavelength selective switches (WSS) configured as ADD and/or DROP modules. The referenced patents disclose wavelength selective switch apparatus and methods comprising an array of fiber coupled collimators that serve as input and output ports for optical signals, a wavelength-separator such as a diffraction grating, a beam-focuser, and an array of channel micromirrors, one micromirror for each spectral channel. In operation, a composite multi-wavelength optical signal (also referred to herein as a “multi-channel optical signal”) from an input port is supplied to the wavelength separator. The wavelength separator spatially separates or demultiplexes the free-space multi-wavelength optical signal into an angular spectrum of constituent spectral channels, and the beam-focuser focuses the spectral channels onto corresponding ones of the channel micromirrors. The channel micromirrors are positioned such that each channel micromirror receives an assigned one of the separated spectral channel beams. The micromirrors are individually controllable and continuously pivotal (or rotatable) so as to reflect the spectral channel beams into selected output ports. This enables each channel micromirror to direct its corresponding spectral channel into any possible output port and thereby switch the spectral channel to any desired output port. Each output port may receive none, one, or more than one of the reflected and so directed spectral channels. Spectral channels may be selectively dropped from a multi-channel signal by switching the channels to different output ports, and new input channels may be selectively added or combined with the original channels to form different multi-wavelength composite signals.
It is also desirable, for a number of reasons, to be able to monitor and control the power in individual spectral channels of the multi-wavelength optical signal. This includes the ability to completely block the power contained in a particular spectral channel. One reason for controlling the power in a channel is to enable “hitless” switching to minimize undesired crosstalk during repositioning of a channel micromirror to direct (“switch”) an input spectral channel beam to a desired output port. During repositioning, the channel micromirror redirects the input spectral channel beam across, i.e., “hits”, intermediate ports, which couples unwanted light into the intermediate ports, and causes crosstalk. Thus, it is desirable either to completely block or to substantially attenuate the power in the beam during switching so that unwanted light coupling is avoided. Another use of monitoring and controlling the optical power of a channel is to cause attenuation of that channel to some predetermined level.
The above-mentioned U.S. patents disclose one approach to power management and hitless switching that employs a spatial light modulator, such as a liquid crystal pixel array, to attenuate or completely blocking the power contained in the spectral channels. Each pixel in the liquid crystal array is associated with one of the spectral channels, and a separate focal plane is created at the location of the liquid crystal array such that a spectral spot corresponding to each channel is located on its associated pixel. Since the voltage applied to the pixel controls the light transmissivity of a pixel, the pixel can be made less transmissive or even opaque to the transmission of light by applying an appropriate voltage, thereby attenuating or completely blocking the power in the spectral channel passing through that pixel. However, this approach has the disadvantage of requiring additional components, including a relay lens system to create a focal plane at the liquid crystal array, the liquid crystal array itself, and electronics to control the liquid crystal array. In addition to the added costs for such additional components, more physical space is needed to accommodate these components, which increases the overall size and complexity of the system.
U.S. Pat. No. 6,549,699 discloses another approach to power management of spectral channels in which the rotation of a channel micromirror about its switching axis (the axis of the parallel to the array of channel micromirrors) is controlled to vary the spatial location of the reflected spectral channel beam relative to its intended output port. Since the amount of power in a spectral channel that is coupled to an output port is a function of the coupling efficiency, a desired power level can be obtained by pivoting the channel micromirror a predetermined angle to decouple the optical beam relative to the output port to attenuate it by an amount corresponding to the desired output power level.
A disadvantage of this latter approach is that decoupling the spectral channel beam spatially repositions the beam along the switching axis. Depending upon the physical spacing of adjacent output ports, a portion of the beam may be cross-coupled into an adjacent output port, causing detrimental cross-talk between the ports. Increasing the physical spacing of the ports to decrease the cross-coupling undesirably increases the physical size of the device. Furthermore, as will be described in detail later, using this approach it is difficult to accurately control the power output levels of spectral channels due to the sensitivity of the coupling to rotation of the channel mirror about the switching axis. To overcome this, wavelength selective switches have been developed that utilize rotation of a channel micromirror about a separate axis (herein referred to as the attenuation axis) to vary the power of a selected beam. However, this approach can lead to a non-uniform attenuation of the passband in the form of side lobes herein referred to as side lobe height. It would be desirable to have a wavelength selective switch that is able to achieve accurate attenuation of separate channels without these passband non-uniformities.
It is to these ends that embodiments of the present invention are directed.